Your body has 100 trillion cells, but only 10 trillion are human. The rest belong to the bacteria, fungi, viruses, and parasites that live on or in us. Some of these tenants are actually beneficial, aiding in the digestion process, for example. The majority of them neither help nor hurt us, but simply co-exist with us. A few species, however, from the cholera bacilli to tapeworms and lice, can be dangerous, and sometimes deadly.